Methane, the major constituent of natural gas, has a higher hydrogen/carbon ratio (H/C) than any other fuel and consequently a higher Research Octane No. than other fuels; 130 for natural gas compared to 87 for unleaded gasoline. Unfortunately, methane cannot be stored at a density as high as other fuels, and thus has an energy density approximately one-third that of gasoline; 11 MJ/L for compressed natural gas at 24.8 MPa (3600 psi) compared to 32 MJ/L for gasoline. Thus a compressed natural gas (CNG) fuel tank would need to be approximately three times larger than a gasoline tank to allow a vehicle the same driving range. The use of CNG has its disadvantages.
The CNG storage tanks must be pressure vessels and are thus constrained in their geometry, but are typically cylindrical. The tanks are also rather heavy, typically about 1 kg/L for steel tanks. Moreover, attainment of  greater than 20.7 MPa (3000 psi) pressure requires costly multi-stage compression.
For these reasons the US Department of Energy has pursued a research program aimed at the development of suitable materials for the storage of natural gas in the physically adsorbed state. Adsorbed natural gas (ANG) is conventionally stored in porous carbon materials at a gas pressure of 3.5 MPa (500 psi). This lower storage pressure reduces the cost of the storage vessel, allows the use of single stage compressors, and represents a lesser safety hazard than the higher pressures used for CNG. The DOE storage target for ANG has been set at 150 V/V, i.e., 150 liters of gas stored per liter of pressure vessel internal volume at standard temperature and pressure (STP=101.325 KPa, 298K).
The four issued U.S. patents below disclose a carbon with comparable storage densities to this invention. They disclose a method of manufacture and product for a wood derived porous carbon in powder form manufactured by chemical activation. They do not teach or disclose the monolith of this invention. U.S. Pat. No. 5,710,092 (Jan. 20, 1998); U.S. Pat. No. 5,416,056 (May 16, 1995); U.S. Pat. No. 5,626,637 (May 6, 1997); U.S. Pat. No. 5,965,483 (Oct. 12, 1999).
The following patents disclose powdered and granular carbons or molecular sieves capable of storing natural gas and methane. They do not teach an electrically conductive carbon fiber based monolith capable of electrical desorption, nor do they disclose enhanced thermal conductivity. U.S. Pat. No. 5,071,820 (Dec. 10, 1991); U.S. Pat. No. 5,094,736 (Mar. 10, 1992); U.S. Pat. No. 5,102,855 (Apr. 7, 1992); U.S. Pat. No. 5,372,619 (Dec. 13, 1994); U.S. Pat. No. 5,292,706 (Mar. 8, 1994); U.S. Pat. No. 5,292,707 (Mar. 8, 1994); U.S. Pat. No. 5,461,023 (Oct. 24, 1995); U.S. Pat. No. 5,614,460 (Mar. 25, 1997); U.S. Pat. No. 5,639,707 (Jun. 17, 1997); U.S. Pat. No. 5,837,741 (Nov. 17, 1998).
The present invention is an adsorbent monolith based on carbon fibers that has improved methane gas storage capabilities. Additionally, the monolithic nature of the storage carbon allows it to exhibit greater thermal conductivity than conventional granular activated carbon or powdered activated carbon storage beds. The storage of methane gas is achieved through the process of physical adsorption in the micropores that are developed in the structure of the adsorbent monolith. The disclosed monolith is capable of storing greater than 150 V/V of methane [i.e.,  greater than 150 STP (101.325 KPa, 298K) volumes of methane per unit volume of storage vessel internal volume] at a pressure of 3.5 MPa (500 psi). At storage capacities of  greater than 150 V/V, natural gas is competitive with compressed natural gas storage at a pressure of 24.8 MPa (3600 psi).